The M3 half-track was used as a command and
control vehicle by headquarters staff elements in the 781st, and a
modified version served as the basis for the self-propelled 81mm mortars
used by the Mortar Platoon in Headquarters Company. Although fast and
relatively mobile, the main drawback of the lightly armored half-tracks
were their open tops, which made them vulnerable to shrapnel pelting
downward from artillery, rocket, or mortar projectiles which may have
detonated above the vehicle. In the heavily-wooded Vosges Mountains, where
the tall trees frequently caused such projectiles to detonate before they
hit the ground, this was an especial weakness for the design.

The self-propelled 81mm version was
designated the M4A1. Its secondary armament consisted of a single
M1919A4 air-cooled .30-caliber MG, but the vehicle otherwise possessed
similar performance characteristics.